ControlMyMonitor - View and modify the settings of your monitor from command line or GUI
NirCmd - NirCmd is a small utility that allows you to do many useful tasks from command-line, without displaying any user interface: change your display settings, turn off your monitor, open the door of your CD-ROM drive, and more...
PangoBright - Screen dimming software that allows you to control the brightness of your laptop screens as well as external monitors.
Yad - Yad (yet another dialog) is a fork of Zenity with many improvements, such as custom buttons...
Dimmer - A very small and free utility for Windows to reduce brightness on LCD/TFT screens.
Zenity - Zenity is a tool that allows you to display GTK dialog boxes in commandline and shell scripts.